U.S. patent application number 14/779554 was filed with the patent office on 2016-02-11 for current collector, electrode structure, battery and capacitor.
This patent application is currently assigned to UACJ CORPORATION. The applicant listed for this patent is FURUKAWA ELECTRIC CO., LTD., UACJ CORPORATION, UACJ FOIL CORPORATION. Invention is credited to Hidekazu Hara, Yukiou Honkawa, Takahiro Iida, Mitsuya Inoue, Takayori Ito, Tsugio Kataoka, Osamu Kato, Yasumasa Morishima, Sohei Saito, Tatsuhiro Yaegashi, Satoshi Yamabe.
Application Number | 20160042878 14/779554 |
Document ID | / |
Family ID | 51624368 |
Filed Date | 2016-02-11 |
United States Patent
Application |
20160042878 |
Kind Code |
A1 |
Kato; Osamu ; et
al. |
February 11, 2016 |
CURRENT COLLECTOR, ELECTRODE STRUCTURE, BATTERY AND CAPACITOR
Abstract
A current collector with high safety which can realize both of a
superior conductivity at normal temperature conditions and a
superior shut down function at high temperature conditions, is
provided. A current collector, including a conductive substrate;
and a resin layer provided on at least one side of the conductive
substrate, is provided. Here, the resin layer is formed with a
paste containing an aggregate of polyolefin-based emulsion
particles; and a conductive material. Further, the aggregate has an
average particle diameter of 0.5 to 5 .mu.m.
Inventors: |
Kato; Osamu; (Chiyoda-ku,
JP) ; Saito; Sohei; (Chiyoda-ku, JP) ;
Honkawa; Yukiou; (Chiyoda-ku, JP) ; Yaegashi;
Tatsuhiro; (Chiyoda-ku, JP) ; Kataoka; Tsugio;
(Kusatsu-shi, JP) ; Inoue; Mitsuya; (Kusatsu-shi,
JP) ; Yamabe; Satoshi; (Kusatsu-shi, JP) ;
Morishima; Yasumasa; (Chiyoda-ku, JP) ; Ito;
Takayori; (Chiyoda-ku, JP) ; Hara; Hidekazu;
(Chiyoda-ku, JP) ; Iida; Takahiro; (Chiyoda-ku,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
UACJ CORPORATION
UACJ FOIL CORPORATION
FURUKAWA ELECTRIC CO., LTD. |
Tokyo
Tokyo
Tokyo |
|
JP
JP
JP |
|
|
Assignee: |
UACJ CORPORATION
Chiyoda-ku, Tokyo
JP
UACJ FOIL CORPORATION
Chuo-ku, Tokyo
JP
FURUKAWA ELECTRIC CO., LTD.
Chiyoda-ku, Tokyo
JP
|
Family ID: |
51624368 |
Appl. No.: |
14/779554 |
Filed: |
March 26, 2014 |
PCT Filed: |
March 26, 2014 |
PCT NO: |
PCT/JP2014/058669 |
371 Date: |
September 23, 2015 |
Current U.S.
Class: |
429/217 ;
361/502 |
Current CPC
Class: |
H01M 2200/106 20130101;
H01M 4/668 20130101; H01G 11/52 20130101; H01M 4/666 20130101; H01G
11/58 20130101; H01M 4/622 20130101; Y02E 60/13 20130101; H01G
11/28 20130101; H01G 11/50 20130101; H01G 11/42 20130101; H01M
4/667 20130101; H01G 11/68 20130101; H01G 11/70 20130101; Y02E
60/10 20130101; H01M 4/661 20130101; H01G 11/06 20130101 |
International
Class: |
H01G 11/28 20060101
H01G011/28; H01G 11/58 20060101 H01G011/58; H01G 11/42 20060101
H01G011/42; H01G 11/52 20060101 H01G011/52; H01M 4/62 20060101
H01M004/62; H01G 11/50 20060101 H01G011/50 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 29, 2013 |
JP |
2013-074635 |
Claims
1. A current collector, comprising: a conductive substrate; and a
resin layer provided on at least one side of the conductive
substrate; wherein the resin layer is formed with a paste
comprising: an aggregate of polyolefin-based emulsion particles;
and a conductive material; and the aggregate has an average
particle diameter of 0.5 to 5 .mu.m.
2. The current collector of claim 1, wherein a surface of the
aggregate is covered with the conductive material by 5 to 90%.
3. The current collector of claim 1, wherein the polyolefin-based
emulsion particles contain at least one resin selected from the
group consisting of a polypropylene resin, a polyethylene resin, a
polypropylene copolymer resin, and a polyethylene copolymer
resin.
4. The current collector of claim 1, further comprising a polymer
coagulant and/or a low molecular coagulant.
5. The current collector of claim 4, wherein: the polymer coagulant
is at least one polymer having a number average molecular weight of
10.times.10.sup.4 or more, selected from the group consisting of
sodium polyacrylate, urethane modified polyether, and sodium
polyacrylate sulfonate; and the low molecular coagulant is at least
one low molecular compound having a number average molecular weight
of 10.times.10.sup.3 or less, selected from the group consisting of
sodium polyacrylate, urethane modified polyether, and sodium
polyacrylate sulfonate.
6. The current collector of claim 1, wherein the conductive
material comprises carbon powders or metal powders.
7. The current collector of claim 1, wherein the conductive
substrate is aluminum, aluminum alloy, or copper.
8. The current collector of claim 1, wherein an AC impedance Zre
measured under 1 Hz when a separator impregnated with an
electrolyte solution is sandwiched in between the current
collectors having an active material layer provided on the resin
layer, the current collectors facing each other, is: 200
.OMEGA.cm.sup.2 or lower at 30.degree. C.; and a maximum resistance
is 400 .OMEGA.cm.sup.2 or higher at 80 to 165.degree. C.
9. An electrode structure comprising the current collector of claim
1.
10. A battery or a capacitor comprising the electrode structure of
claim 9.
Description
TECHNICAL FIELD
[0001] The present invention relates to current collectors,
electrode structures, batteries, and capacitors.
BACKGROUND
[0002] Regarding lithium ion batteries in the vehicle and the like,
a high speed charge/discharge characteristics (high rate
characteristics) is required at usual usage, and a so-called shut
down function (PTC function) to terminate charge/discharge
automatically and safely is required when an accident such as
malfunction occurs. With respect to the former requirement, a
technique to minimize the grain size of the active material and a
technique to form a conductive layer onto the current collector has
been known. On the other hand, with respect to the latter
requirement, a system to improve the safety of the battery has been
made. For example, a safety valve is used to prevent the inner
pressure from increasing, and a structure to cut off the current
when heat generation occurs is provided by incorporating a PTC
(Positive Temperature Coefficient) element. Here, the PTC element
is an element of which resistance value increases along with the
increase in temperature. Regarding batteries, a technique to
provide the shut down function to a separator has been known. The
separator fuses at high temperature, and thus micropores are
blocked. Accordingly, ionic conduction is blocked, thereby
terminating the electrode reaction under over-heated circumstances.
However, there are cases where the shut down by the separator is
incomplete and thus the temperature increases to above the melting
point of the separator, and cases where the temperature increase in
the external surroundings result in the meltdown of the separator.
Such cases would result in an internal short-circuit. Then, the
shut down function of the separator can no longer be counted on,
and the battery would be in the state of thermal runaway.
[0003] Accordingly, a technique which provides charge/discharge
characteristics during usual usage and improves safety when
accidents such as malfunction occur, has been suggested. For
example, Patent Literature 1 discloses a positive electrode current
collector prepared by adhering a sheet-like conductive polymer (50
.mu.m thickness) onto an aluminum net (20 .mu.m thickness), the
sheet-like conductive polymer having a PTC characteristics of 5
S/cm conductivity at room temperature and 5 .mu.S/cm conductivity
at a working temperature of 120.degree. C. In addition, it is
described that the sheet-like conductive polymer used here is
prepared by mixing 30 wt % of polyethylene with 70 wt % of carbon
black (paragraph 0048 of Patent Literature).
[0004] In addition, Patent Literature 2 discloses of uniformly
coating a conductive paste onto both sides of the expanded metal of
aluminum or copper, using a die coater or a gravure coater,
followed by drying of the paste, thereby obtaining a current
collector having a conductive layer (0.5 .mu.m thickness) formed
thereon. Here, the conductive paste is prepared by adding 35 g of
crystalline polyethylene resin having a melting point of
110.degree. C. and 30 g of acetylene black as the carbon-based
conductive material to 270 g of an N-methyl-2-pyrrolidone (NMP)
solution of polyvinylidene difluoride (PVDF) (13% solids), followed
by kneading using a planetary mixer. Subsequently, 440 g of NMP is
further added to dilute the conductive paste (paragraph 0029 of
Patent Literature 2).
[0005] In addition, Patent Literature 3 discloses of mixing the
acetylene black as the conductive material and polyethylene having
a softening point of 120.degree. C. as the binding polymer with a
weight ratio of 10:1, followed by addition of suitable amount of
carboxymethyl cellulose as a thickener to give a paste mixture.
Subsequently, the mixture is coated onto both sides of the aluminum
foil having a thickness of 10 .mu.m, as the positive electrode
current collector. The mixture is coated with a thickness of 5
.mu.m or less. Then, the coating is dried to obtain a resistive
layer (lines 1 to 6, page 13 of Patent Literature 3).
[0006] In addition, in Patent Literature 4, a coating having fine
particles dispersed in the binder resin is formed. Here, the fine
particles are prepared by crushing an electron conducting material
containing a conductive filler and a resin, the electron conducting
material showing higher resistance as the temperature rises.
Further, this literature mentions that the fine particles function
so as to show higher resistance as the temperature rises.
CITATION LIST
Patent Literature
Patent Literature 1: JP H10-241665A
Patent Literature 2: JP 2001-357854A
Patent Literature 3: WO 2002/54524A
Patent Literature 4: JP 4011635B
SUMMARY OF THE INVENTION
Technical Problem
[0007] However, regarding the conventional techniques described in
the afore-mentioned literatures, they still had room for further
improvement, in terms of the following, and thus were problematic
in providing secure safety.
[0008] First of all, in Patent Literatures 1 to 3, since
polyvinylidene difluoride and polyethylene are thermoplastic
resins, there are cases where the thermoplastic resins fuse when
the temperature reaches above 100.degree. C. during the active
material coating process, thereby resulting in a condition
different from the condition before fusing. Therefore, the
temperature during manufacture of the lithium ion secondary
batteries, lithium ion capacitors and the like cannot exceed
100.degree. C., thereby resulting in cases where the productivity
is low.
[0009] Secondly, in Patent Literature 3, when the current collector
was used for the lithium ion secondary batteries, lithium ion
capacitors and the like, the so called high rate characteristics of
the high speed charge/discharge was not sufficient. Therefore, the
current collector was not suitable for high speed charge/discharge
under usual conditions.
[0010] Thirdly, in Patent Literature 4, since the conductive
fillers (conductive material) were dispersed in the resin, there
was a defect in that the resistance value cannot be made
sufficiently high.
[0011] The present invention has been made by taking the
afore-mentioned circumstances into consideration. An object of the
present invention is to provide a current collector having high
safety, which can achieve both of superior conductivity under
normal temperature conditions and superior shut down function under
high temperature conditions.
Solution to Problem
[0012] According to the present invention, a current collector,
comprising: a conductive substrate; and a resin layer provided on
at least one side of the conductive substrate, is provided. Here,
the resin layer is formed with a paste comprising: an aggregate of
polyolefin-based emulsion particles; and a conductive material.
Further, the aggregate has an average particle diameter of 0.5 to 5
.mu.m.
[0013] According to such current collector, since the current
collector uses an aggregate of the polyolefin-based emulsion
particles and the average particle diameter of such aggregate is
0.5 to 5 .mu.m, both of the superior conductivity under normal
temperature conditions and superior shut down function under high
temperature conditions can be achieved.
[0014] According to the present invention, an electrode structure
comprising the afore-mentioned current collector is obtained. In
addition, according to the present invention, a battery or a
capacitor comprising the afore-mentioned electrode structure is
obtained.
[0015] According to such electrode structure, battery or capacitor,
since the afore-mentioned current collector is used, both of the
superior conductivity under normal temperature conditions and
superior shut down function under high temperature conditions can
be achieved.
Effect of the Invention
[0016] According to the present invention, both of the superior
conductivity under normal temperature conditions and superior shut
down function under high temperature conditions can be
achieved.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a cross-sectional view showing a structure of a
current collector according to one embodiment of the present
invention.
[0018] FIG. 2 is a cross-sectional view showing a structure of an
electrode structure according to one embodiment of the present
invention.
[0019] FIG. 3 is a schematic diagram showing the coating condition
of the surface of the polyolefin-based emulsion particles covered
with the conductive material, used in one embodiment of the present
invention.
[0020] FIG. 4 is a schematic diagram showing the mechanism of PTC
function realization when the aggregate of the polyolefin-based
emulsion particles according to one embodiment of the present
invention is used.
[0021] FIG. 5 is a schematic view showing a condition of inside the
resin layer of the electrode structure under normal temperature
conditions, according to one embodiment of the present invention (a
case where conductive material is added after aggregating the
polyolefin-based emulsion particles by using the polymer
coagulant).
[0022] FIG. 6 is a schematic view showing a condition of inside the
resin layer of the electrode structure under normal temperature
conditions, according to one embodiment of the present invention (a
case where conductive material is added after aggregating the
polyolefin-based emulsion particles by using the polymer coagulant
and low molecular coagulant).
[0023] FIG. 7 is a schematic view showing a condition of inside the
resin layer of the electrode structure under normal temperature
conditions, according to one embodiment of the present invention (a
case where the polyolefin-based emulsion particles are aggregated
by using the polymer coagulant, after conductive material is
added).
[0024] FIG. 8 is a schematic view showing a condition of inside the
resin layer of the electrode structure under normal temperature
conditions, according to one embodiment of the present invention (a
case where the polyolefin-based emulsion particles are aggregated
by using the polymer coagulant and low molecular coagulant, after
conductive material is added).
DESCRIPTION OF EMBODIMENTS
[0025] Hereinafter, the embodiments of the present invention will
be explained with reference to the drawings. Here, in all of the
drawings, the same symbols are provided for the similar
constitutional elements, and the explanations for them are omitted
where applicable. In addition, "A to B" in the present
specification shall mean "A or more and B or less".
<Entire Structure>
[0026] FIG. 1 is a cross-sectional view showing the structure of
the current collector of the present embodiment. The current
collector 100 of the present embodiment comprises a resin layer 105
having conductivity provided on at least one side of the conductive
substrate 103.
[0027] FIG. 2 is a cross-sectional view showing the structure of
the electrode structure prepared by using the current collector of
the present embodiment. On the resin layer 105 of the current
collector 100 of the present embodiment, an active material layer
115 is formed. Accordingly, the electrode structure 117 suitable
for the non-aqueous electrolyte batteries such as lithium ion
secondary batteries and the like can be prepared.
<Circumstances of the Invention>
[0028] FIG. 3 is a schematic diagram showing the coating condition
of the surface of the polyolefin-based emulsion particles being
covered with the conductive material, used in the present
embodiment. In order to solve the afore-mentioned problem, the
present inventors have used the polyolefin-based emulsion particles
125 having superior dispersibility in an aqueous solution as the
resin structuring the paste being coated onto the conductive
substrate 103. However, since the particle diameter of the
polyolefin-based emulsion particles 125 were 0.1 .mu.m or larger
and less than 0.4 .mu.m, the amount of deformation caused by
thermal expansion was small. Accordingly, the cut off of the
conductive path (cut off of the connection between the conductive
materials 121) by temperature increase was not sufficient.
[0029] Accordingly, the present inventors have added a crosslinker
to this paste to crosslink the polyolefin-based emulsion particles
125, thereby forming a large crosslinked product. However, this
resulted in increase in the resistance at normal temperature due to
generation of gas and water by the crosslinking reaction. On the
other hand, when the polyolefin-based emulsion particles 125 were
aggregated to form a large aggregated product, the resistance at
normal temperature was maintained low since there was no generation
of gas and water. Then, the present inventors have made an
investigation on the PTC function for a case where the
polyolefin-based emulsion particles 125 were aggregated to obtain a
large aggregated product. Accordingly, the present inventors found
that both of the superior conductivity under normal temperature
conditions and superior shut down function under high temperature
conditions can be achieved, resulting in accomplishment of the
present invention.
<Mechanism of PTC Function Realization>
[0030] FIG. 4 is a schematic diagram showing the mechanism of PTC
function realization when the aggregate of the polyolefin-based
emulsion particles of the present embodiment is used. The resin
layer 105 of the current collector 100 of the present embodiment
comprises a paste including an aggregate 131 of the
polyolefin-based emulsion particles 125 and a conductive material
121. Here, the coating weight when this paste is coated onto the
conductive substrate 103 is preferably 0.5 to 20 g/m.sup.2. In
addition, the average particle diameter of the aggregate is 0.5 to
5 .mu.m.
[0031] The conductive material 121 is distributed on the surface or
in the gap of the polyolefin-based emulsion particles 125 or the
aggregate 131 of the polyolefin-based emulsion particles 125 and
are in contact with each other during normal usage. Here, the
conductive material 121 do not get inside the polyolefin-based
emulsion particles 125.
[0032] The resin layer 105 of the present embodiment realizes the
PTC function when an accident occurs. The particle diameter of the
polyolefin-based emulsion particles 125 itself is as small as 0.1
.mu.m or larger and smaller than 0.4 .mu.m. However, the particle
diameter of the aggregate 131 of the polyolefin-based emulsion
particles 125 is in a suitable range of 0.5 to 5 .mu.m, thereby
providing large deformation by thermal expansion. Accordingly, cut
off of the conductive path (cut off of the connection between
conductive material 121) by temperature increase is sufficient.
That is, the aggregate 131 of the polyolefin-based emulsion
particles 125 expands by thermal expansion, thereby cutting off the
network of the conductive material 121 adhered onto the aggregate
131. Accordingly, the resistance is increased.
[0033] As shown in FIGS. 5 to 8 mentioned later, in the present
embodiment, the conductive material 121 efficiently (with minimum
amount) forms the conductive path under normal temperature
conditions. Therefore, superior conductivity is achieved under
normal temperature conditions. On the other hand, when the
temperature rises, cut off of the conductive path tends to occur by
the expansion of the aggregate 131 of the polyolefin-based emulsion
particles 125. Therefore, in the present embodiment, sufficient
battery property and PTC function can be obtained with a relatively
small amount of conductive material 121 when compared with the case
where the olefin-based resin which dissolves in an organic solvent
is used. That is, in the present embodiment, a current collector
100 which can achieve both of superior conductivity under normal
temperature conditions and superior shut down function under high
temperature conditions, can be realized.
<Explanation of Each of the Constituents>
(1. Conductive Substrate)
[0034] The current collector 100 of the present embodiment is
prepared by coating a paste onto at least one side of the
conductive substrate 103. As the conductive substrate 103,
conductive substrate 103 known as various metal foils for
non-aqueous electrolyte batteries, electrical double layer
capacitors, or lithium ion capacitors can generally be used.
Specifically, various metal foils for the positive electrode and
negative electrode can be used, such as foils of aluminum, aluminum
alloy, copper, stainless steel, and nickel. Among these, foils of
aluminum, aluminum alloy, and copper are preferable in terms of the
balance between conductivity and cost.
[0035] There is no particular limitation regarding the thickness of
the conductive substrate 103. Here, the thickness is preferably 5
.mu.m or more and 50 .mu.m or less. When the thickness is less than
5 .mu.m, the strength of the foil would be insufficient, thereby
resulting in cases where formation of the resin layer becomes
difficult. On the other hand, when the thickness exceeds 50 .mu.m,
other constituents, especially the active material layer or the
electrode layer need be made thin to compensate such thickness,
when such conductive substrate is used for the non-aqueous
electrolyte batteries, and electrical storage devices such as
electrical double layer capacitors and lithium ion capacitors.
Accordingly, there would be a case where necessary capacity cannot
be obtained. Here, the thickness of the conductive substrate 103
can be in the range of two values selected among 5, 10, 15, 20, 25,
30, 35, 40, 45, and 50 .mu.m.
(2. Polyolefin-Based Emulsion Particles)
[0036] FIG. 3 is a schematic diagram showing the structure of the
polyolefin-based emulsion particles used in the present embodiment.
There is no particular limitation regarding the polyolefin-based
emulsion particles 125 used in the present embodiment. For example,
the polyolefin-based emulsion particles preferably contain at least
one resin having a large linear expansion coefficient and a
superior adhering property selected from the group consisting of a
polypropylene resin, a polyethylene resin, a polypropylene
copolymer resin, and a polyethylene copolymer resin. Especially
preferably, polypropylene resin, polyethylene resin,
polyethylene-polypropylene block copolymer resin,
polyethylene-polypropylene graft copolymer resin and the like can
be used as the polyolefin-based emulsion particles. In addition,
one of these resins can be used alone, or two or more resins can be
used in combination.
[0037] In addition, the polyolefin-based resin structuring the
afore-mentioned polyolefin-based emulsion particles 125 can be
modified with a carboxylic acid (or a carboxylic acid anhydride),
or can be not modified with a carboxylic acid (or a carboxylic acid
anhydride). Here, the resin component used for the resin layer 105
of the present embodiment can comprise only the afore-mentioned
polyolefin-based emulsion particles 125, or can contain other resin
components. However, it is unfavorable to use a solution type
polyolefin-based resin (in organic solvent) which does not form
emulsion particles, since it is difficult for the resistance to
increase when the PTC function is realized.
[0038] Here, there is no particular limitation regarding the
carboxylic acid (or carboxylic acid anhydride) for modifying the
afore-mentioned polyolefin-based resin. For example, it is
preferable to use maleic acid, acrylic acid, pyromellitic acid,
citric acid, tartaric acid, oxylaic acid, mellitic acid,
terephthalic acid, adipic acid, fumaric acid, itaconic acid,
trimellitic acid, or isophthalic acid. Here, either one of these
acids can be an acid anhydride.
[0039] The average particle diameter of the polyolefin-based
emulsion particles itself (primary particles) used in the present
embodiment is preferably 0.1 .mu.m or more and less than 0.4 .mu.m.
The primary particles mentioned here are particles formed by
dispersing the polyolefin-based resin in water and the like. When
the particle size of the primary particles is less than 0.1 .mu.m,
the particle size of the secondary aggregate of the
polyolefin-based emulsion particles 125 would only be less than 0.5
.mu.m. On the other hand, when the particle size of the primary
particles exceeds 0.4 .mu.m, the particle size of the aggregate
would become too large, and thus defects such as increase in
resistance at room temperature and unstable coating conditions
would occur, resulting in failure to obtain the desired current
collector.
(3. Aggregate)
[0040] The aggregate 131 of the polyolefin-based emulsion particles
125 formed in the resin layer 105 of the present embodiment has a
larger structure (secondary particles or particles of higher
dimensions) by aggregation of the plurality of polyolefin-based
emulsion particles 125 themselves (primary particles). Here, this
aggregate can easily be formed by using a polymer coagulant and/or
low molecular coagulant described later. However, coagulant need
not necessarily be used.
[0041] The average particle diameter of the aggregate 131 is 0.5 to
5 .mu.m, preferably 1 to 5 .mu.m, and more preferably 2 to 5 .mu.m.
When the average particle diameter of the aggregate 131 is less
than 0.5 .mu.m, the amount of deformation by the thermal expansion
at elevated temperature would not be sufficient. On the other hand,
when the average particle diameter of the aggregate 131 exceeds 5
.mu.m, the coating would become too thick, resulting in defects
such as resistance increase at room temperature and unstable
emulsion solution which would cause separation of the components.
In addition, since the aggregate 131 is an aggregate of primary
particles, there are many fine concave and convex portions compared
with the primary particles (the contact portion of the primary
particles become the concave and convex portions), and thus the
conductive material 121 easily adhere. Accordingly, when the
particle diameter is the same, the aggregate 131 has an advantage
in that it can lower the resistance at normal usage. Here, the
average particle diameter of the aggregate 131 can be calculated by
measuring the particle diameter distribution of a paste prepared
without formulating the conductive material 121, using a particle
size analyzer. As the particle size analyzer, commercially
available apparatuses utilizing the dynamic light scattering
method, laser diffraction/scattering method, image imaging method,
and the like can suitably be used.
(4. Conductive Material)
[0042] The polyolefin-based emulsion particles 125 used for the
resin layer 105 of the present embodiment need be formulated with a
conductive material 121 in order to provide electron conductivity.
As the conductive material 121 used in the present embodiment,
known carbon powders and metal powders can be used. Among these,
carbon powders are preferable. As the carbon powders, acetylene
black, Ketjen black, furnace black, carbon nanotubes, and various
graphite particles can be used. In addition, the average particle
diameter of the conductive material 121 is preferably 100 nm or
smaller. When the particle diameter is too large, separation tends
to occur during storage of the coating, and thus the coating would
become uneven when coated, thereby making it difficult to cut off
the conductive path when the temperature is raised. The average
particle diameter of the conductive material 121 is more preferably
60 nm or smaller. In order to disperse the conductive material 121
in the paste, a planetary mixer, a ball mill, a homogenizer and the
like can be used.
[0043] There is no particular limitation regarding the formulation
amount of the conductive material 121 of the present embodiment.
Here, in order to realize the desired PTC function with high
safety, it is preferable that the safety to realize the PTC
function can be maintained with a small amount of the binder resin
compared with that for the normal carbon coatings and active
material layer.
[0044] In particular, with respect to 100 parts by mass of the
resin component of the polyolefin-based emulsion particles 125, the
formulation amount of the conductive material 121 is preferably 5
to 50 parts by mass, more preferably 6 to 45 parts by mass, and
further preferably 8 to 40 parts by mass. When the formulation
amount of the conductive material 121 is 5 parts by mass or less,
the volume resistivity of the resin layer 105 becomes high,
resulting in cases where sufficient conductivity as the current
collector 100 cannot be obtained. On the other hand, when the
formulation amount of the conductive material 121 exceeds 50 parts
by mass, the connection between the conductive materials 121 cannot
be cut off even when the volume is expanded, resulting in cases
where sufficient resistance cannot be obtained. Here, the
formulation amount of the conductive material 121 can be in the
range of two values selected among 5, 6, 7, 8, 9, 10, 15, 20, 25,
30, 35, 40, 45, and 50 parts by mass.
[0045] There is no particular limitation regarding the coverage
ratio of the surface of the aggregate 131 of the polyolefin-based
emulsion particles 125 of the present embodiment being covered with
the conductive material 121. In order to achieve both of the
superior conductivity under normal temperature conditions and
superior shutdown function under high temperature conditions, the
coverage ratio is preferably 5 to 90%, more preferably 10 to 80%,
and further preferably 15 to 70%. When the coverage ratio is less
than 5%, characteristics of the battery or the capacitor such as
conductivity can become insufficient regarding the usage under
normal temperature conditions. On the other hand, when the coverage
ratio exceeds to 90%, there are cases where the conductive path
cannot be cut off sufficiently when the temperature is raised.
Here, the coverage ratio can be in the range of two values selected
among 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75,
80, 85, and 90%.
[0046] Here, the coverage ratio can be measured by first coating
the conductive substrate 103 with a paste, and then performing an
observation of a cross section of the coating regarding the resin
layer 105 formed with the paste. The ratio of the surface of the
aggregate 131 of the emulsion-particles 125 being covered with the
conductive material 121 is the coverage ratio by the conductive
material 121.
[0047] In the present embodiment, the coverage ratio by the
conductive material 121 can be measured by exposing the cross
section using an ion milling, and then obtaining the ratio of the
surface of the aggregate 131 being covered with the conductive
material 121. Here, as the position of observation, the coating is
cut at 10 portions to expose the cross sections, and then 10
arbitrary portions for each of the cross sections are selected (100
in total). The average of the coverage ratio obtained from each of
the observation made at each of the selected portion is then
calculated.
(5. Formulation of Paste)
[0048] The paste used in the present embodiment can be formulated
with an arbitrary method. For example, the paste can be formulated
by a method as described below.
(1) Embodiment Having Conductive Material Mainly on the Surface of
the Aggregate (Surface of Secondary Particles)
(1-1) Only Polymer Coagulant Used
[0049] FIG. 5 is a schematic view showing the condition of inside
the resin layer of the electrode structure under normal temperature
conditions, according to the present embodiment (a case where
conductive material is added after aggregating the polyolefin-based
emulsion particles by using the polymer coagulant). First, the
aggregate 131 of 0.2 to 5 .mu.m is formed by adding the polymer
coagulant 123 to the polyolefin-based emulsion particles 125 (for
example, water-borne emulsion such as polypropylene), followed by
mixing. Here, when only the polymer coagulant 123 is used, the
average particle diameter of the aggregate 131 tends to become
small.
[0050] Then, conductive material 121 is further added, followed by
mixing, thereby allowing the conductive material 121 to adhere onto
the surface of the aggregate 131 to achieve the coverage ratio of 5
to 90%. The coverage ratio can be adjusted by adjusting the
formulation amount of the conductive material 121. The paste thus
obtained is coated onto the conductive substrate 103, and then the
coating is dried to form the resin layer 105. Subsequently, the
active material layer 115 is formed onto the resin layer 105 to
prepare the electrode structure 117. Regarding the electrode
structure 117 of this embodiment, there were many cut offs in the
conductive path among the conductive materials 121 caused by the
expansion of the aggregate 131 of the polyolefin-based emulsion
particles at high temperature conditions. Accordingly, the shut
down effect was large.
(1-2) Polymer Coagulant and Low Molecular Coagulant Used
[0051] FIG. 6 is a schematic view showing a condition inside the
resin layer of the electrode structure under normal temperature
conditions, according to the present embodiment (a case where
conductive material is added after aggregating the polyolefin-based
emulsion particles by using the polymer coagulant and low molecular
coagulant). First, the aggregate 131 of 0.5 to 5 .mu.m is formed by
adding the polymer coagulant 123 and the low molecular coagulant
127 to the polyolefin-based emulsion particles 125 (for example,
water borne emulsion such as polypropylene), followed by mixing.
Here, since the polymer coagulant 123 and the low molecular
coagulant 127 are used in combination, the average particle
diameter of the aggregate 131 tends to become large.
[0052] Then, conductive material 121 is further added, followed by
mixing, thereby allowing the conductive material 121 to adhere onto
the surface of the aggregate 131 to achieve the coverage ratio of 5
to 90%. The coverage ratio can be adjusted by adjusting the
formulation amount of the conductive material 121. The paste thus
obtained is coated onto the conductive substrate 103, and then the
coating is dried to form the resin layer 105. Subsequently, the
active material layer 115 is formed onto the resin layer 105 to
prepare the electrode structure 117. Regarding the electrode
structure 117 of this embodiment, there were many cut offs in the
conductive path among the conductive materials 121 caused by the
expansion of the aggregate 131 of the polyolefin-based emulsion
particles at high temperature conditions. Accordingly, the shut
down effect was large.
(2) Form Having Conductive Material Also Inside the Aggregate
(Surface of Primary Particles)
(2-1) Only Polymer Coagulant Used
[0053] FIG. 7 is a schematic view showing a condition inside of the
resin layer of the electrode structure under normal temperature
conditions, according to the present embodiment (a case where the
polyolefin-based emulsion particles are aggregated by using the
polymer coagulant after conductive material is added). First, the
conductive material 121 is added to the polyolefin-based emulsion
particles 125 (for example, water borne emulsion such as
polypropylene), followed by mixing. Accordingly, the conductive
material 121 is adhered onto the surface of the polyolefin-based
emulsion particles 125 so that the coverage ratio would be 5 to
90%. The coverage ratio can be adjusted by adjusting the
formulation amount of the conductive material 121.
[0054] Then, polymer coagulant 123 is further added, followed by
mixing, thereby forming the aggregate 131 of 0.5 to 5 .mu.m. Here,
since only the polymer coagulant 123 is used, the average particle
diameter of the aggregate 131 tends to become small. Then, since
the aggregate 131 is obtained by aggregating the polyolefin-based
emulsion particles 125 being covered with the conductive material
121 by a coverage ratio of 5 to 90%, the coverage ratio of the
aggregate 131 by the conductive material 121 would be also 5 to
90%. The paste thus obtained is coated onto the conductive
substrate 103, and then the coating is dried to form the resin
layer 105. Subsequently, the active material layer 115 is formed
onto the resin layer 105 to prepare the electrode structure 117.
Regarding the electrode structure 117 of this embodiment, there are
many conductive paths among the conductive materials 121 since the
conductive material 121 exist also in the aggregate 131 (surface of
the primary particles). Accordingly, the resistance under normal
temperature conditions can be suppressed.
(2-2) Polymer Coagulant and Low Molecular Coagulant Used
[0055] FIG. 8 is a schematic view showing a condition inside of the
resin layer of the electrode structure under normal temperature
conditions, according to the present embodiment (a case where the
polyolefin-based emulsion particles are aggregated by using the
polymer coagulant and low molecular coagulant after conductive
material is added). First, the conductive material 121 is added to
the polyolefin-based emulsion particles 125 (for example, water
borne emulsion such as polypropylene), followed by mixing.
Accordingly, the conductive material 121 is adhered onto the
surface of the polyolefin-based emulsion particles 125 so that the
coverage ratio would be 5 to 90%. The coverage ratio can be
adjusted by adjusting the formulation amount of the conductive
material 121.
[0056] Then, polymer coagulant 123 is further added, followed by
mixing, thereby forming the aggregate 131 of 0.5 to 5 .mu.m. Here,
since the polymer coagulant 123 and the low molecular coagulant are
used in combination, the average particle diameter of the aggregate
131 tends to become large. Then, since the aggregate 131 is
obtained by aggregating the polyolefin-based emulsion particles 125
being covered with the conductive material 121 by a coverage ratio
of 5 to 90%, the coverage ratio of the aggregate 131 by the
conductive material 121 would be also to 90%. The paste thus
obtained is coated onto the conductive substrate 103, and then the
coating is dried to form the resin layer 105. Subsequently, the
active material layer 115 is formed onto the resin layer 105 to
prepare the electrode structure 117. Regarding the electrode
structure 117 of this embodiment, there are many conductive paths
among the conductive materials 121 since the conductive material
121 exist also in the aggregate (surface of the primary particles).
Accordingly, the resistance under normal temperature conditions can
be suppressed.
(6. Coagulant)
[0057] As the coagulant of the present embodiment added to the
paste to form the aggregate 131, any coagulant can be used so long
as the coagulant can aggregate a plurality of polyolefin-based
emulsion particles 125 to form a larger structure. When the polymer
coagulant 123 is used, although there is no particular limitation
regarding the polymer coagulant 123, it is preferable that the
polymer coagulant 123 contains at least one polymer selected from
the group consisting of sodium polyacrylate, urethane modified
polyether, and sodium polyacrylate sulfonate. Here, such polymer
has been confirmed of its superior coagulating effect as described
in the following Examples.
[0058] In addition, taking into consideration that the polymer
coagulant 123 entangle with the emulsion particles 125 so as to
stretch a net over the emulsion particles 125, thereby forming the
aggregate 131 having an average particle diameter of 0.5 to 5
.mu.m, it is preferable that the number average molecular weight of
the polymer coagulant 123 is 10.times.10.sup.4 or more, more
preferably 15.times.10.sup.4 or more, and especially preferably
20.times.10.sup.4 or more.
[0059] Further, from the same reasons, the number average molecular
weight of the polymer coagulant 123 is preferably
100.times.10.sup.4 or less, more preferably 80.times.10.sup.4 or
less, and further preferably 50.times.10.sup.4 or less. When the
number average molecular weight of the polymer coagulant 123 is
less than 10.times.10.sup.4, the average particle diameter of the
aggregate 131 tends to be less than 0.5 .mu.m.
[0060] On the other hand, when the number average molecular weight
of the polymer coagulant 123 exceeds 100.times.10.sup.4, it would
become difficult to dissolve the polymer coagulant 123 in aqueous
solution. Accordingly, it would become difficult to disperse the
polymer coagulant, thereby failing to form the aggregate particles
or forming aggregates with a size exceeding 5 .mu.m, which are
undesirable. Here, the number average molecular weight can be in
the range of two values selected among 10.times.10.sup.4,
15.times.10.sup.4, 20.times.10.sup.4, 25.times.10.sup.4,
30.times.10.sup.4, 35.times.10.sup.4, 40.times.10.sup.4,
45.times.10.sup.4, 50.times.10.sup.4, 55.times.10.sup.4,
60.times.10.sup.4, 65.times.10.sup.4, 70.times.10.sup.4,
75.times.10.sup.4, 80.times.10.sup.4, 85.times.10.sup.4,
90.times.10.sup.4, 95.times.10.sup.4, and 100.times.10.sup.4.
[0061] On the other hand, when the low molecular coagulant 127 is
used, although there is no particular limitation regarding the low
molecular coagulant 127, it is preferable that the low molecular
coagulant 127 contains at least one low molecular compound selected
from the group consisting of sodium polyacrylate, urethane modified
polyether, and sodium polyacrylate sulfonate. Here, such low
molecular compound has been confirmed of its superior coagulating
effect as described in the following Examples.
[0062] In addition, taking into consideration that the low
molecular coagulant 127 connects each of the emulsion particles
125, and further adhere the connected emulsion particles 125
tightly, it is preferable that the number average molecular weight
of the low molecular coagulant 127 is 10.times.10.sup.3 or less,
more preferably 8000 or less, and especially preferably 7000 or
less. When the number average molecular weight of the low molecular
coagulant 127 is in the range of more than 10.times.10.sup.3 and
less than 10.times.10.sup.4, the low molecular coagulant 127 would
get caught in between the emulsion particles 125 as a foreign
substance, providing distance between the emulsion particles 135,
thereby resulting in defects such as increase in the resistance at
room temperature. The polymer coagulant 123 and the low molecular
coagulant 127 can be used alone, or can be used in combination.
[0063] There is no particular limitation regarding the formulation
amount of the polymer coagulant 123 and/or low molecular coagulant
127. Here, it is preferable that the formulation amount of each of
the polymer coagulant 123 and/or low molecular coagulant 127 is
0.0001 to 0.1 parts by mass, more preferably 0.001 to 0.01 parts by
mass with respect to 100 parts by mass of the resin component of
the polyolefin-based emulsion particles 125. When the formulation
amount is less than 0.0001 parts by mass, there are cases where
sufficient amount of deformation due to thermal expansion cannot be
obtained when the temperature is raised. In addition, when the
formulation amount of the polymer coagulant 123 and/or low
molecular coagulant 127 with respect to 100 parts by mass of the
resin component of the polyolefin-based emulsion particles 125
exceeds 0.01 parts by mass, the aggregation would proceed to far,
and thus the expansion occur in the surface direction rather than
the thickness direction, thereby resulting in cases where the
conductive path cannot be cut off sufficiently when the temperature
is raised.
(7. Resin Layer)
[0064] FIG. 1 is a cross-sectional view showing a structure of a
current collector according to the present embodiment. The current
collector 100 of the present embodiment has a resin layer 105 using
the afore-mentioned paste. When the resin layer 105 is used in the
positive electrode, this resin layer 105 is preferably provided on
the conductive substrate 103 as the resin layer 105 having the PTC
function. In such case, the resin layer 105 is provided separately
from the active material layer 115.
[0065] There is no particular limitation regarding the method for
forming the resin layer 105 having conductivity used in the present
embodiment. Preferably, the polyolefin-based emulsion particles
125, conductive material 121, and the polymer coagulant 123 and/or
low molecular coagulant 127 are mixed in water or in aqueous
solution to prepare a composition for current collector (paste),
and then this composition for current collector (paste) is coated
onto the conductive substrate 103. In the coating process, a roll
coater, a gravure coater, a slit die coater and the like can be
used.
[0066] In the current collector 100 of the present embodiment, the
coating amount (coating weight) of the composition for current
collector (paste) for forming the resin layer 105 is preferably 0.5
to 20 g/m.sup.2, more preferably 1 to 10 g/m.sup.2, and especially
preferably 2 to 5 g/m.sup.2. When the coating amount is less than
0.5 g/m.sup.2, there would be cases where resistance does not
increase when the temperature is raised. On the other hand, when
the coating amount exceeds 20 g/m.sup.2, there would be cases where
the resistance under normal temperature conditions (30.degree. C.),
becomes too high. Here, the coating amount can be in the range of
two values selected among 0.5, 1, 2.5, 5, 10, and 20 g/m.sup.2.
[0067] After coating the composition for current collector (paste)
onto the conductive substrate 103, baking is performed to cure the
composition for current collector, thereby forming the resin layer
105. There is no particular limitation regarding the baking
temperature. Here, for example, the baking temperature is
preferably 80 to 240.degree. C. When the baking temperature is
below 80.degree. C., the curing degree would be insufficient,
resulting in cases where the adhesion of the conductive substrate
with the resin layer 105 is insufficient. On the other hand, when
the baking temperature exceeds 240.degree. C., the resin may melt
depending on the type of polyolefin-based resin used, resulting in
change in the arrangement of the conductive material. This can
cause problems since the PTC function cannot be realized. Here, the
baking temperature can be in the range of two values selected among
80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 220,
and 240.degree. C.
[0068] There is no particular limitation regarding the baking
period. For example, the baking period is preferably 5 to 200
seconds. When the baking period is less than 5 seconds, the curing
degree would be insufficient, resulting in cases where the adhesion
of the conductive substrate with the resin layer 105 is
insufficient. On the other hand, when the baking period exceeds 200
seconds, the productivity would become low, while improvement in
performance cannot be obtained, which would be meaningless. Here,
the baking period can be in the range of two values selected among
5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 120, 140, 160, 180, and
200 seconds.
(8. Electrode Structure for Battery)
[0069] FIG. 2 is a cross-sectional view showing a structure of an
electrode structure prepared by using the current collector of the
present embodiment. The electrode structure 117 can be obtained by
forming the active material layer 115 on the resin layer 105 of the
current collector 100 of the present embodiment. Then, a separator
impregnated with an electrolyte solution is sandwiched in between
this electrode structure 117 as the positive electrode and another
electrode structure separately prepared as the negative electrode,
thereby preparing a non-aqueous electrolyte battery such as a
lithium ion secondary battery.
[0070] Here, as the active material layer 115 provided to the
electrode structure 117 of the present embodiment, the one used for
the non-aqueous electrolyte batteries can suitably be used. For
example, when the electrode structure 117 for the positive
electrode is prepared, a current collector 100 using an aluminum
alloy foil as the conductive substrate 103 is coated with a paste
prepared by dispersing LiCoO.sub.2, LiMnO.sub.4, LiNiO.sub.2 and
the like as the active material and carbon black such as acetylene
black as the conductive material in PVDF or water dispersion type
PTFE as the binder. The paste thus coated is dried to form the
active material layer 115.
[0071] When the electrode structure 117 for the negative electrode
is prepared, a current collector 100 using copper foil as the
conductive substrate 103 is coated with a paste prepared by
dispersing black lead, graphite, mesocarbon microbeads and the like
as the active material in CMC (carboxymethyl cellulose) as the
thickener followed by mixing with SBR (styrene butadiene rubber) as
the binder. The paste thus coated is dried to form the active
material layer 115.
(9. Electrode Structure for Capacitor)
[0072] The electrode structure 117 can be obtained by forming the
electrode material layer 115 on the resin layer 105 of the current
collector 100 of the present embodiment. Then, a separator
impregnated with an electrolyte solution is sandwiched in between
this electrode structure 117 as the positive electrode and another
electrode structure 117 as the negative electrode, thereby
preparing a capacitor for electrical double layer capacitor,
lithium ion capacitor and the like.
[0073] Here, as the electrode material 115, the one conventionally
used for the electrode material of the electrical double layer
capacitor and lithium ion capacitor can be used. For example,
carbon powders such as active charcoal and black lead, and carbon
fibers can be used. As the binder, for example, PVDF
(polyvinylidene difluoride), SBR, water dispersion type PTFE and
the like can be used.
(10. Performance of Electrode Structure)
[0074] The electrode structure 117 was used as the positive
electrode, and another electrode structure 117 was used as the
negative electrode. A separator impregnated with the electrolyte
solution was sandwiched in between these electrode structures and
the AC impedance Zre was measured under 1 Hz. It is preferable that
the resistance is 200 .OMEGA.cm.sup.2 or lower at 30.degree. C.,
and the maximum resistance is 400 .OMEGA.cm.sup.2 or higher at
80.degree. C. or higher and 165.degree. C. or lower. When the AC
impedance Zre exceeds 200 .OMEGA.cm.sup.2 at 30.degree. C., the
high rate characteristics during high speed charge/discharge is
insufficient, and thus the electrode structure is not suitable for
high speed charge/discharge under normal conditions. On the other
hand, when the AC impedance Zre shows maximum resistance of lower
than 400 .OMEGA.cm.sup.2 at 80.degree. C. or higher and 165.degree.
C. or lower, the shut down function at elevated temperature would
be insufficient, thereby failing to prevent thermal runaway.
[0075] As for the separator, a film having a polyolefin microporous
and non-woven fabric can be used for example. As for the
non-aqueous electrolyte, there is no limitation so long as there is
no side reaction such as decomposition when used within a voltage
range for non-aqueous electrolyte battery, electrical double layer
capacitor, and lithium ion capacitor. For example, as the positive
ion, tertiary ammonium salts such as tetraethyl ammonium salt,
triethylmethyl ammonium salt, and tetrabutyl ammonium salt can be
used. As the negative ion, hexafluoro phosphate salt, tetrafluoro
borate salt, and perchloric salt can be used.
[0076] As the non-aqueous solvent, aprotic solvents such as
carbonates, esters, ethers, nitriles, sulfonic acids, and lactones
can be used. For example, one or more non-aqueous solvents selected
from the group consisting of ethylene carbonate (EC), propylene
carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC),
ethylmethyl carbonate (EMC), 1,2-dimethoxy ethane, 1,2-diethoxy
ethane, tetrahydrofuran, 2-methyl tetrahydrofuran, dioxiane,
1,3-dioxolane, diethylene glycol dimethyl ether, ethylene glycol
dimethyl ether, acetonitrile, propionitrile, nitromethane,
N,N-dimethylformamide, dimethyl sulfoxide, sulforane,
.gamma.-butyrolactone can be used.
[0077] The embodiments of the present invention were described with
reference to the Drawings. Here, such embodiments are merely an
exemplification, and thus various structures other than the ones
mentioned above can be adopted.
Examples
[0078] Hereinafter, the present invention will be described in
detail with reference to Examples. However, the present invention
shall not be limited to these.
(1) Preparation of Coating and Application Thereof
[0079] As shown in Table 1, the coatings (pastes) were prepared by
performing two-stage agitation of agitation process 1 and agitation
process 2. Each of the agitation process were performed using a
disper, with a rotation number of 1000 rpm and an agitation period
of 60 minutes. (Here, in Tables 1 and 2, PP and PE means
polypropylene and polyethylene, respectively. Further, the addition
amount of the conductive material and the coagulant are given as
the addition amount with respect to 100 parts by mass of the base
resin.) The coating (paste) was coated onto one side of an aluminum
foil (JIS A1085) having a thickness of 15 .mu.m using a bar coater
by a coating amount (coating weight, weight per unit area) as shown
in Table 3. Subsequently, the coating was subjected to baking for
24 seconds with a peak metal temperature (PMT) of 110.degree. C. to
give a current collector.
TABLE-US-00001 TABLE 1 base resin agitation process 1 addition
addition addition amount conductive amount amount (parts by
material (parts by coagulant (parts by type of resin mass) type
mass) type mass) Example 1 PP 100 acetylene black 30 none -- 2 PP
100 acetylene black 30 none -- 3 acrylic acid modified PP 100
acetylene black 30 none -- 4 acrylic acid modified PP 100 acetylene
black 30 none -- 5 acrylic acid modified PE 100 acetylene black 15
none -- 6 acrylic acid modified PE 100 acetylene black 40 none -- 7
maleic acid modified PP 100 none -- sodium polyacrylate 0.001
(molecular weight 2 .times. 10.sup.5) 8 maleic acid modified PP 100
none -- sodium polyacrylate 0.01 (molecular weight 2 .times.
10.sup.5) urethane modified polyether (molecular weight 5000) 9 PE
100 acetylene black 20 none -- 10 ethylene-glycidyl ether copolymer
100 acetylene black 20 none -- 11 acrylic acid modified PE-PP
copolymer 100 acetylene black 20 none -- 12 maleic acid modified
PE-PP copolymer 100 acetylene black 20 none -- 13 PP 100 acetylene
black 20 none -- 14 PP 100 acetylene black 20 none -- 15 acrylic
acid modified PE 100 acetylene black 5 none -- 16 acrylic acid
modified PE 100 acetylene black 100 none -- 17 acrylic acid
modified PE 100 acetylene black 8 none -- Comparative 18 acrylic
acid modified PP (90) 100 acetylene black 20 none -- Example
melamine (10) 19 PP 100 acetylene black 20 none -- 20 acrylic resin
100 acetylene black 20 none --
TABLE-US-00002 TABLE 2 agitation process 2 conductive material
coagulant addition addition amount amount (parts by (parts by type
mass) type mass) Example 1 none -- sodium polyacrylate 0.001
(molecular weight 2 .times. 10.sup.5) 2 none -- sodium polyacrylate
0.001 (molecular weight 2 .times. 10.sup.5) 3 none -- urethane
modified polyether 0.0001 (molecular weight 3 .times. 10.sup.5) 4
none -- sodium polyacrylate 0.01 (molecular weight 2 .times.
10.sup.5) 0.01 urethane modified polyether (molecular weight 5000)
5 none -- sodium polyacrylate 0.001 (molecular weight 2 .times.
10.sup.5) 6 none -- sodium polyacrylate 0.001 (molecular weight 2
.times. 10.sup.5) 7 acetylene black 20 none -- 8 acetylene black 20
none -- 9 none -- urethane modified polyether 0.001 (molecular
weight 3 .times. 10.sup.5) 10 none -- urethane modified polyether
0.001 (molecular weight 3 .times. 10.sup.5) 11 none -- sodium
polyacrylate sulfonate 0.001 (molecular weight 25 .times. 10.sup.4)
12 none -- sodium polyacrylate sulfonate 0.001 (molecular weight 25
.times. 10.sup.4) 13 none -- sodium polyacrylate 0.001 (molecular
weight 2 .times. 10.sup.5) 14 none -- sodium polyacrylate 0.001
(molecular weight 2 .times. 10.sup.5) 15 none -- sodium
polyacrylate 0.001 (molecular weight 2 .times. 10.sup.5) 16 none --
sodium polyacrylate 0.001 (molecular weight 2 .times. 10.sup.5) 17
none -- sodium polyacrylate 0.001 (molecular weight 2 .times.
10.sup.5) Comparative 18 none -- sodium polyacrylate 0.001 Example
(molecular weight 2 .times. 10.sup.5) 19 none sodium polyacrylate
0.2 (molecular weight 2 .times. 10.sup.5) 20 none -- sodium
polyacrylate 0.001 (molecular weight 2 .times. 10.sup.5)
TABLE-US-00003 TABLE 3 maximun resistance at average particle
coverage ratio resistace 80.degree. C. or higher coating weight of
diamter of by conductive at 30.degree. C. and 165.degree. C. or
coating (g/m.sup.2) aggregate (.mu.m) material (%)
(.OMEGA.cm.sup.2) lower (.OMEGA.cm.sup.2) Example 1 0.6 3.2 51 46
410 2 20 3.1 49 198 5820 3 3.1 0.5 50 118 520 4 3.2 5.0 52 164 6140
5 2.9 3.0 11 128 7400 6 3.0 3.1 90 36 2370 7 3.2 3.2 60 124 8540 8
3.1 4.9 61 142 10120 9 3.3 2.8 53 68 5640 10 3.2 2.5 52 90 4400 11
3.3 2.6 52 96 4360 12 3.1 2.8 51 98 4430 13 0.3 3.3 49 40 380 14 22
3.1 48 210 6230 15 3.0 3.3 3 205 8200 16 3.0 3.1 100 30 390 17 3.1
3.1 5 198 7970 Comparative 18 2.9 0.3 48 150 350 Example 19 25 8 52
390 6700 20 3.0 2.9 53 42 240
<Evaluation Method>
(1) Coating Amount (Coating Weight, Weight Per Unit Area)
[0080] The coated foil was cut into 100 mm squares, and the weight
was measured. After removing the coating, the weight was measured
again, and the coating amount (coating weight, weight per unit
area) was calculated as a balance. The results of measurement are
shown in Table 2.
(2) Particle Diameter of Aggregate
[0081] The particle diameter of the aggregate was obtained by
measuring the particle diameter distribution of the coating (paste)
prepared without adding the conductive material, using the particle
size analyzer. Here, a laser diffraction/scattering particle size
distribution analyzer LA-950V2 available from HORIBA, Ltd. was used
as the particle size analyzer to calculate the volume average
particle diameter.
(3) Coverage Ratio by Conductive Material
[0082] The coverage ratio by the conductive material was measured
as follows. First, the coating (paste) was coated, followed by
exposure of the cross section of the coating by ion milling.
Subsequently, the cross section of the resin layer was observed
using a field emission type scanning microscope available from
Hitachi High-Technologies Corporation. The ratio of the surface of
the aggregate being covered with the conductive material was taken
as the coverage ratio by the conductive material. Here, regarding
the position of observation, the coating was cut at 10 portions to
expose the cross sections, and then 10 arbitrary portions for each
of the cross sections were selected (100 in total). The average of
the coverage ratio obtained from each of the observation made at
each of the selected portions was then calculated.
(4) Measurement of Resistance
[0083] The active material paste (active material: LMO, binder:
PVDF, conductive assistant: acetylene black) was coated onto the
current collector prepared as above. The coating was then dried,
pressed, and punched out by 1615.95 mm.phi., thereby obtaining an
electrode. A separator (cellulose-based material) impregnated with
an electrolyte solution (composition: 1 mol/L LiBF.sub.4 in EC:EMC
(1:3 V/V %) was sandwiched in between two of these electrodes so
that the coated surfaces face each other, thereby obtaining a cell.
The cell thus obtained was subjected to AC impedance measurement
with an amplitude of 30 mV and a frequency of 1 Hz, while raising
the temperature in the oven from ambient temperature (30.degree. C.
or lower) to 165.degree. C. at the rate of 5.degree. C./min, using
VersaSTAT4 available from Princeton Applied Research. Here, the Zre
at 1 Hz was taken as the resistance. The AC impedance Zre shows the
resistance component of the impedance.
[0084] Here, the one having lower resistance at 30.degree. C. is
superior in charge/discharge characteristics, and can be applied
for batteries with high output. As a criterion, the resistance of
200 .OMEGA.cm.sup.2 or lower would enable usage in general
batteries. On the other hand, higher shut down effect can be
obtained when the maximum resistance at 80 to 165.degree. C. is
higher. As a criterion, maximum resistance of 400 .OMEGA.cm.sup.2
or higher would allow realization of the shut down effect in
general batteries in case of overcharge.
<Discussion on Results>
[0085] From the above experimental results, it can be said that
when the polyolefin-based emulsion particles are used as the base
resin and the coating weight of the coating, average particle
diameter of the aggregate, and coverage ratio of the aggregate by
the conductive material are in the preferable range, the initial
resistance can be lowered and the PTC magnification can be
increased.
[0086] On the other hand, when the coating weight is less than 0.3
g/m.sup.2, the resistance cannot sufficiently be raised when the
temperature is raised, and when the bases weight exceeds 22
g/m.sup.2, the resistance at 30.degree. C. would become too
high.
[0087] In addition, when the average particle diameter of the
aggregate of the emulsion particles is less than 0.5 .mu.m, the
amount of deformation by thermal expansion would be insufficient at
elevated temperature. On the other hand, when the average particle
diameter exceeds 5 .mu.m, the coating would become too thick,
thereby resulting in defects such as increase in the resistance at
room temperature and separation of the composition due to unstable
emulsion solution.
[0088] Further, when the coverage ratio of the aggregate by the
conductive material is less than 5%, the maximum resistance at
elevated temperature would become high, however, the battery
characteristics regarding normal usage would be inferior. On the
other hand, when the coverage ratio exceeds 90%, the maximum
resistance at elevated temperature would become low, resulting in
low cut off effect of the conductive path.
[0089] In addition, when the coagulant and the crosslinker are used
in combination, the resistance at 30.degree. C. would become too
high.
[0090] Further, when the base resin other than the polyolefin-based
ones was used, the maximum resistance at to 165.degree. C. was low
and the shut down function was insufficient.
[0091] Accordingly, the present invention has been described with
reference to the Examples. Here, these Examples are merely
exemplification. The person having ordinary skill in the art shall
understand that various alteration can be made, and that such
alterations are within the scope of the present invention.
EXPLANATION OF SYMBOLS
[0092] 100: current collector [0093] 103: conductive substrate
[0094] 105: resin layer [0095] 115: active material layer [0096]
117: electrode structure [0097] 121: conductive material [0098]
123: polymer coagulant [0099] 125: polyolefin-based emulsion
particles [0100] 127: low molecular coagulant [0101] 131:
aggregate
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